The present invention is directed to processes for curing polyimides and to thermal ink jet printheads containing polyimide components prepared by this process. More specifically, the present invention is directed to processes for curing polyimides by reacting polyimide precursors with borane. One embodiment of the present invention is directed to a process which comprises reacting a polyimide precursor with borane. Another embodiment of the present invention is directed to an ink jet printhead which comprises (i) an upper substrate with a set of parallel grooves for subsequent use as ink channels and a recess for subsequent use as a manifold, the grooves being open at one end for serving as droplet emitting nozzles, (ii) a lower substrate in which one surface thereof has an array of heating elements and addressing electrodes formed thereon, and (iii) a thick film layer deposited on the surface of the lower substrate and over the heating elements and addressing electrodes and patterned to form recesses therethrough to expose the heating elements and terminal ends of the addressing electrodes, said thick film layer comprising the reaction product of a polyimide precursor with borane, the upper and lower substrates being aligned, mated, and bonded together to form the printhead with the grooves in the upper substrate being aligned with the heating elements in the lower substrate to form droplet emitting nozzles. Yet another embodiment of the present invention is directed to a process for preparing an ink jet printhead which comprises the steps of:
(a) providing a photopatternable polyimide precursor polymer; PA1 (b) depositing a thick film layer comprising the photopatternable polymer onto a lower substrate in which one surface thereof has an array of heating elements and addressing electrodes having terminal ends formed thereon, said photopatternable polymer being deposited onto the surface having the heating elements and addressing electrodes thereon; PA1 (c) exposing the thick film layer to actinic radiation in an imagewise pattern, wherein the unexposed areas correspond to areas of the lower substrate having thereon the heating elements and the terminal ends of the addressing electrodes; PA1 (d) removing the photopatternable polymer from the unexposed areas, thereby forming recesses in the thick film layer, said recesses exposing the heating elements and the terminal ends of the addressing electrodes; PA1 (e) reacting the photopatternable polymer in the exposed areas with borane; PA1 (f) providing an upper substrate with a set of parallel grooves for subsequent use as ink channels and a recess for subsequent use as a manifold, the grooves being open at one end for serving as droplet emitting nozzles, and PA1 (g) aligning, mating, and bonding the upper and lower substrates together to form a printhead with the grooves in the upper substrate being aligned with the heating elements in the lower substrate to form droplet emitting nozzles. PA1 (a) providing a photopatternable polyimide precursor polymer; PA1 (b) depositing a thick film layer comprising the photopatternable polymer onto a lower substrate in which one surface thereof has an array of heating elements and addressing electrodes having terminal ends formed thereon, said photopatternable polymer being deposited onto the surface having the heating elements and addressing electrodes thereon; PA1 (c) exposing the thick film layer to actinic radiation in an imagewise pattern, wherein the unexposed areas correspond to areas of the lower substrate having thereon the heating elements and the terminal ends of the addressing electrodes; PA1 (d) removing the photopatternable polymer from the unexposed areas, thereby forming recesses in the thick film layer, said recesses exposing the heating elements and the terminal ends of the addressing electrodes; PA1 (e) reacting the photopatternable polymer in the exposed areas with borane; PA1 (f) providing an upper substrate with a set of parallel grooves for subsequent use as ink channels and a recess for subsequent use as a manifold, the grooves being open at one end for serving as droplet emitting nozzles; and PA1 (g) aligning, mating, and bonding the upper and lower substrates together to form a printhead with the grooves in the upper substrate being aligned with the heating elements in the lower substrate to form droplet emitting nozzles.
Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream type. There are different types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The other type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink in the immediate vicinity to vaporize almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The surface of the printhead encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet equipment and processes are well known and are described in, for example, U.S. Pat. No. 4,601,777, U.S. Pat. No. 4,251,824, U.S. Pat. No. 4,410,899, U.S. Pat. No. 4,412,224, U.S. Pat. No. 4,532,530, and U.S. Pat. No. 4,774,530, the disclosures of each of which are totally incorporated herein by reference.
In ink jet printing, a printhead is usually provided having one or more ink-filled channels communicating with an ink supply chamber at one end and having an opening at the opposite end, referred to as a nozzle. These printheads form images on a recording medium such as paper by expelling droplets of ink from the nozzles onto the recording medium. The ink forms a meniscus at each nozzle prior to being expelled in the form of a droplet. After a droplet is expelled, additional ink surges to the nozzle to reform the meniscus.
In thermal ink jet printing, a thermal energy generator, usually a resistor, is located in the channels near the nozzles a predetermined distance therefrom. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. The rapidly expanding vapor bubble pushes the column of ink filling the channel towards the nozzle. At the end of the current pulse the heater rapidly cools and the vapor bubble begins to collapse. However, because of inertia, most of the column of ink that received an impulse from the exploding bubble continues its forward motion and is ejected from the nozzle as an ink drop. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
Ink jet printheads include an array of nozzles and may, for example, be formed of silicon wafers using orientation dependent etching (ODE) techniques. The use of silicon wafers is advantageous because ODE techniques can form structures, such as nozzles, on silicon wafers in a highly precise manner. Moreover, these structures can be fabricated efficiently at low cost. The resulting nozzles are generally triangular in cross-section. Thermal ink jet printheads made by using the above-mentioned ODE techniques typically comprise a channel plate which contains a plurality of nozzle-defining channels located on a lower surface thereof bonded to a heater plate having a plurality of resistive heater elements formed on an upper surface thereof and arranged so that a heater element is located in each channel. The upper surface of the heater plate typically includes an insulative layer which is patterned to form recesses exposing the individual heating elements. This insulative layer is referred to as a "pit layer" and is sandwiched between the channel plate and heater plate. For examples of printheads employing this construction, see U.S. Pat. No. 4,774,530 and U.S. Pat. No. 4,829,324, the disclosures of each of which are totally incorporated herein by reference. Additional examples of thermal ink jet printheads are disclosed in, for example, U.S. Pat. No. 4,835,553, U.S. Pat. No. 5,057,853, and U.S. Pat. No. 4,678,529, the disclosures of each of which are totally incorporated herein by reference.
U.S. Pat. No. 4,601,777 (Hawkins et al.), the disclosure of which is totally incorporated herein by reference, discloses fabricating processes for ink jet printheads, each printhead being composed of two parts aligned and bonded together. One part is a substantially flat substrate which contains on the surface thereof a lineal array of heating elements and addressing electrodes, and the second part is a substrate having at least one recess anisotropically etched therein to serve as an ink supply manifold when the two parts are bonded together. A lineal array of parallel grooves are formed in the second part, so that one end of the grooves communicates with the manifold recess and the other ends are open for use as ink droplet expelling nozzles. Many printheads can be made simultaneously by producing a plurality of sets of heating elements array with their addressing electrodes on a silicon wafer and by placing alignment marks thereon at predetermined locations. A corresponding plurality of sets of channels and associated manifolds are produced in a second silicon wafer and, in one embodiment, alignment openings are etched thereon at predetermined locations. The two wafers are aligned via the alignment openings and alignment marks, then bonded together and diced into many separate printheads.
U.S. Pat. No. 4,678,529 (Drake et al.), the disclosure of which is totally incorporated herein by reference, discloses a method of bonding ink jet printhead components together by coating a flexible substrate with a relatively thin, uniform layer of an adhesive having an intermediate non-tacky curing stage with a shelf life with around one month for ease of alignment of the parts and ease of storage of the components having the adhesive thereon. Transferring about half of the adhesive layer on the flexible substrate to the high points or lands of one of the printhead components within a predetermined time of the coating of the flexible substrate is achieved by placing it in contact therewith and applying a predetermined temperature and pressure to the flexible substrate prior to peeling it from the printhead component. This process causes the adhesive to fail cohesively in the liquid state, assuring that about half of the thickness of the adhesive layer stays with the flexible substrate and is discarded therewith, leaving a very thin uniform layer of adhesive on the printhead component lands. The transferred adhesive layer remaining on the printhead component enters an intermediate non-tacky curing stage to assist in subsequent alignment of the printhead components. The printhead components are aligned and the adhesive layer cured to complete fabrication of the printhead.
Known thermal ink jet printheads frequently include a barrier layer or pit layer of a polyimide material. Many polyimide barrier layers are unable to withstand prolonged contact with the materials commonly employed in thermal ink jet inks. Aqueous inks having a pH of 8 or more are particularly likely to cause hydrolysis and delamination of the polyimide barrier layer in a thermal ink jet printhead. While not being limited to any particular theory, it is believed that the partially cured polyimide layer is subject to hydrolytic attack when contacted with basic inks.
Accordingly, while known compositions and processes are suitable for their intended purposes, a need remains for improved thermal ink jet printheads. In addition, a need remains for thermal ink jet printheads in which the barrier layer exhibits reduced lamination upon exposure to inks. Further, there is a need for thermal ink jet printheads with barrier layers having increased resistance to hydrolysis. Additionally, there is a need for thermal ink jet printheads which exhibit improved adhesion of the heater wafer to the channel wafer subsequent to prolonged exposure to inks. There is also a need for processes for preparing improved thermal ink jet printheads with these advantages. A further need exists for processes for curing polyimide components without the need for application of heat. Additionally, a need remains for processes for curing polyimide components wherein shrinkage of the material during curing is reduced or eliminated. A further need exists for processes for curing polyimide films without reduced stresses induced by heat cure during the imidization step.